The present invention relates to a projection aligner, and more particularly to a novel projection aligner which can control a luminosity (or intensity) and the distribution (or uniformity) thereof.
In forming an integrated circuit on a semiconductor wafer, the so-called photolithographic process is performed. In this process, a projection aligner is utilized as an apparatus for printing the pattern of a photo-mask on the surface of the wafer. Since the projection aligner focuses the pattern of the photo-mask on the wafer surface and exposes a photoresist film on the wafer surface to light by the use of an optical system, it has the advantage of versatile printing when compared with a contact type apparatus which holds a photo-mask and a wafer in close contact and then executes exposure to light. On the other hand, however, the projection type is disadvantageous in that nonuniformity in the luminosity of the illuminating light and nonuniformity in the distribution of the luminosity are prone to directly affect the pattern image on the wafer. To the end of forming a pattern image of proper luminosity and uniform luminosity distribution, accordingly, it is required to strictly control the luminosity of the illuminating light and the distribution thereof.
To this end, heretofore, it has merely been conducted as one of the operations of the photolithographic process that an operator measures the luminosities at the positions of the projection aligner near the wafer and adjusts the various parts of the aligner on the basis of the measured results so as to make the luminosities uniform. Therefore, even when the luminosity or the luminosity distribution has deviated in the course of the operation or during any other preparatory operation, it is difficult to find out the deviation. Ultimately, the photoresist film is exposed to light of improper luminosity or luminosity distribution, and the exposed dimensions do not fall within ratings, resulting in the problem that a nonconforming article arises to worsen the available percentage.
On the other hand, in the pattern exposure of the photolithographic process, in order to achieve good printing by the use of a projection aligner which focuses a mask pattern image on a wafer through a projecting optical system, a luminosity on the wafer needs to be held stable. To this end, it is considered to furnish the projection aligner with a luminosity control device. The luminosity control device should ideally be so constructed that light actually projected on the wafer surface is detected by a sensor and that the quantity of light of a lamp or the like is controlled by feedback on the basis of a result obtained by comparing the detected value with a reference value. In actuality, however, the construction is difficult for such a reason that the mask pattern shades the sensor. It is accordingly the real situation that the sensor is inevitably installed in another place within the optical system.
It is therefore considered to arrange the sensor near the lamp or near the reflector of an illuminating optical system. Such position, however, is a position on an upper stream side (on a side closer to the lamp which is a light source for exposure) when viewed from the entire optical system. It is impossible to precisely grasp the quantity of the actual light which reaches the wafer after starting from here and passing through a large number of further optical elements.
Meanwhile, in order to feedback-control the quantity of light of the lamp, the light quantity of the lamp needs to be precisely detected. It can accordingly be proposed to dispose also a sensor for light adjustment, called a "monitor under the lamp", at a position near the lamp and to send the output of the sensor to the aforementioned comparing system for the feedback control of the light quantity of the lamp. The light adjusting sensor, however, involves the problems that a complicated structure is necessary for eliminating the influences of lamp heat etc. and that an attendant amplifier is necessitated to increase the cost.